Global Navigation Satellite Systems (GNSS) allow navigation services including automatic route calculation from a current location to a destination location and guiding a driver of a vehicle to that destination with real time instructions in conjunction with a visual display of route segments as the vehicle progresses along the route.
Several different approaches for position location are known, including for example, terrestrial methods, GNSS methods, and hybrids of terrestrial and satellite-based methods. One such GNSS is the Global Positioning System (GPS), developed and operated by the U.S. Defense Department, which includes a constellation of at least 24 well-spaced satellites that orbit the earth at a distance of approximately 20,000 kilometers.
The satellites transmit GPS signals comprising very precise location parameters and timing signals that are received by wireless device processors allowing a processor to determine their respective three dimensional positions and velocities.
Each satellite transmits Almanac information, which includes information regarding the coarse location of all satellites in the constellation. Each satellite further transmits Ephemeris information, which includes a higher accuracy version of data pertaining to its own orbit, as tracked and reported by tracking stations on earth. At any one time, a GPS receiver receives Ephemeris for only those satellites currently being tracked. Not only are signals from different satellites acquired, tracked, and lost over a course of hours, Ephemeris transmitted from each satellite changes every hour to guarantee accurate approximation of the current orbital parameters.
One of the key parameters of a commercial GPS system is Time-To-First-Fix (TTFF). TTFF is defined as a time period from the moment a GPS unit is powered on to the moment it starts providing continuous position and velocity updates based upon the processing of signals received from the tracked GPS satellites. TTFF comprises two major parts, time-to-detect and acquire satellite signals and time-to-collect the Ephemeris. The signal acquisition is generally speeded up by using multi-channel receivers that can simultaneously search multiple frequencies.
Currently, GPS receivers can detect and lock onto the GPS satellite signal in a matter of a few seconds. However, the second part, collecting the GPS Ephemeris data, still takes about 30 seconds with a high signal-to-noise ratio (SNR), or an even longer period of time in the case of low SNR. This long processing time results in a user having to wait at least an extra 30 seconds before getting guidance instructions from a GPS navigation system. This wait time is often inconvenient for users who wish to know navigation instructions from the moment the navigation system is turned on.
Because the transmitted Ephemeris parameters provide accurate approximation of the true orbital parameters only for a limited time, Ephemeris information can only be used for 2-4 hours before it must be recollected from either the same or newly acquired satellites. Therefore, even for users who use their GPS navigation system every day and have latest collected Ephemeris for all tracked satellites stored in the non-volatile memory, if the GPS device was powered off for a few hours, data must be recollected and the users must wait more than 30 seconds for the device to process new Ephemeris information.
This problem has been partially resolved by the use of terrestrial systems that integrate wireless networks, i.e., cellular communication systems, and GPS satellites. GPS assisted systems rely upon terrestrial systems with base stations to collect Ephemeris and re-transmits the data via the cellular communication systems or other radio-link to the navigation device.
However, such GPS aided systems are not available in all locations, are not supported by many available GPS receivers, and are not toll free. Accordingly there is a need for apparatus and methods that allows users of non-GPS aided navigation systems to enjoy the full benefits of GPS guidance immediately after the GPS signal has been acquired, or when current Ephemeris is otherwise unavailable.